1. Field of the Invention
This invention relates to mobile radio telephones. The invention has particular, although not exclusive, relevance to mobile radio telephones for use in the Pan European cellular radio network, commonly referred to as the Groupe Speciale Mobile (GSM) system.
Mobile radio telephones have to meet conflicting requirements of compact size, long battery life and high transmitter power. Furthermore, to comply with the GSM specification, mobile radio telephones must fit into one of a number of specified classes. These classes link transmitter power capability to the dynamic range performance of the receiver, and thus any increase in the power for the transmitter will change the specification of the performance of the receiver.
2. Description of the Prior Art
Manufacturers of mobile radio telephones for use in the GSM system may produce a range of products which meet the requirements of mobile radio telephones for use in the GSM system. At one extreme, the telephone may take the form of a hand-portable unit which is designed to be as small and light as possible, this necessarily being at the expense of transmitter power and receiver performance. At the other extreme the telephone may be mounted in a car, the telephone using the car battery, or a large independent NiCd battery to give high transmitter output power, the result thus being a larger unit which is less portable. In order to derive the benefits of both of these extremes, a hand-portable unit may be connected to a unit called a power booster which is mounted, for example, in the boot of a car. This then enables the hand-portable unit to have the performance of a telephone mounted in a car when the hand-portable unit is connected to the booster unit. However, it is necessary to know the effect of the booster unit on the receiver in the hand-portable unit, so as to be able to accurately measure the magnitude of the received signal. In known mobile radio telephones, for example as shown in European Patent Application EP-0338834, signals are routed between the transceiver incorporated in the hand-portable unit and the power booster at the final signal frequency. The main unknown in the prediction of the effect of the booster unit on the receiver in the hand-portable unit is thus the length, and hence the losses produced in the cable connecting the hand-portable unit and the booster through which radio frequency signals at the carrier frequency are transmitted.